U.S. Pat. Nos. 4,066,447 and 4,144,102 disclose a low CTE material commercially available as Incoloy.RTM. alloy 908. (Incoloy is a trademark of the Into family of companies). The alloy includes about 4% chromium, about 3% niobium, about 1.5% titanium, about 1% aluminum, about 49% nickel, and the balance iron. Cobalt is an optional element ranging from 0 to about 31%. Cobalt containing versions are usually destined for aerospace applications--turbine gas seals and rings. Cobalt-free (or low level variations) exhibit satisfactory CTE values. However, they are employed in situations where cobalt containing materials are undesirable such as in neutron flux environments.
U.S. Pat. No. 4,785,142, which utilizes a cobalt-free version of Incoloy alloy 908, discloses a superconducting cable sheath including about 46-50% nickel, about 3-6% chromium, about 2.5-3.5% niobium, about 1.25%-1.65% titanium, about 0.8-1.2% aluminum, and the balance iron.
Alloys described in the referenced prior art patents are well suited for wrought alloy applications required for superconductor sheathing in the form of seamless tubes or thin-walled autogenous welded tubes. However, these alloys display Nb rich, Laves-type phases that segregate upon solidification during welding. Such phases lead to undesirable reduced ductility and toughness in the weldments. There exists a need for weldments with improved toughness in these applications. The improvement in toughness, however, must be realized with a minimum sacrifice of alloy strength.